A narrative review on the use of near-infrared spectroscopy to monitor bladder volume and in vitro validation approaches

Background and Objective: Current guidelines for patients with neurogenic bladder (NB) dysfunction suggest the use of self-intermittent catheterization with adherence to catheterization timers. Due to biorhythmic variability, unpredictable voiding times may occur. As a result, many patients abstain from extended social or work activities, turn to more secluded lifestyles, and generally experience a decrease in quality of life. Being able to know when to void is essential for patients with NB dysfunction. To solve the problem of variable void timings, wearable devices using near-infrared spectroscopy (NIRS) have been emergent and showed potential for bladder volume monitoring. Therefore, in this review, we provided a comprehensive overview of research which implemented NIRS for bladder volume monitoring, and discussed how the researchers validated their device by in-vitro methods and suggested a potential validation approach. Methods: A literature search using PubMed and Google Scholar was conducted on February 2023. Publications involving bladder volume monitoring incorporating NIRS technique with in-vitro validation was considered for review. Key Content and Findings: Due to the novelty of NIRS being applied to bladder monitoring, there are a few possibilities to effectively validate this technique through in-vitro methods. Ballistics gel, which has been proven to be a versatile material for applications involving ultrasound, could be a suitable material when constructing a bladder phantom for in-vitro validation of NIRS technology. Conclusions: By outlining a more standardized in-vitro validation model based on ballistics gel, we hope to facilitate development in this field towards a more accurate and robust NIRS-based bladder monitoring device.